1. Field of the Invention
The present invention relates to a method for controlling an expiratory valve in a ventilator.
2. Description of the Prior Art
In normal circumstances in respiratory care, the patient is 10 allowed to exhale as normally as possible, sometimes against an elevated positive end expiratory pressure (PEEP). The tubes (the tracheal tube in particular) and devices (e.g. a dehumidifier, bacterial filter and the ventilator's expiratory valve in particular) in the path of flow of expired gas pose a resistance to expiration. The patient is forced to overcome this unnatural resistance, which can become tiring.
One way to reduce such resistance is to open the expiratory valve to a maximum for a specific period of time. East German Patentschrift 293 268 describes one such method for controlling a ventilator, wherein the expiratory valve consists of an on/off valve with only two positions, fully open or fully closed.
This known control of the expiratory valve causes the expiratory valve to open at the onset of expiration. It is kept open for a specific period of time and then closed. The pressure (end pressure) on the valve (on the patient side) then corresponds to the pressure in the patient's lungs. The period of time in which the valve is kept open for the next consecutive breathing cycles is set according to the difference between the determined end pressure (actual value) and a preset value for PEEP (reference value). The time the valve is kept open is increased if the measured value exceeds the reference value. The time the valve is kept open is reduced if the measured value is less than the reference value. In this way, a convergence toward the reference value is obtained.
A disadvantage of this known control system is that the patient risks exposure to an end pressure that is less than PEEP during an initial phase of treatment (when maintenance of PEEP is particularly important in preventing the collapse of alveoli in the lungs)
Another disadvantage of this known control system is that the patient is subjected to varying end pressures, at least during the adjustment phase of treatment, since an end pressure greater than the target PEEP pressure could develop.
An additional disadvantage of this known control system is that the patient's lungs and the tubing do not constitute a static system. Any change in the patient's physical position could alter parameters for the gas mechanics of the lungs/tubing system, for which the control system is unable to compensate. In a worst case scenario, this could result in an end pressure much lower (or higher) than the reference value.
Yet another disadvantage is the fact that bias flows cannot be used, since the known valve is an on/off valve. Bias flows have the advantage of making flow-triggering possible for the patient.